A VFSA Method of Travel Time Inversion for Layer Interval Anisotropy Estimation Using Walk-away VSP
Donghong Pei, Bruce E. Cornish, R. Zhou, Dan Quinn, R. Williamson · Proceedings · 2011
We developed a new implementation of VFSA as an alternative method of travel time inversion for Thomsen’s parameters using walk-away VSP data. Velocity along the symmetry axis and TTI axis orientation must be known. Two synthetic tests show that the inverted parameters are within 10% of their true model values. The sensitivity analysis shows that ε is more dependent on ray paths in the horizontal direction or group angles close to 900 , while δ is more affected by ray paths with 550 incident angles. This implies that walk-away surveys designed for this purpose may require longer source-receiver offsets, e.g. the ratio of offset to depth may be equal to or greater than 1. Consequently, head and / or guided waves may interfere with accurate first arrival time determination and care is required in picking the proper event.